Quantum properties usually show up only at the atomic level. Those properties include the quantization of energies, wavelike dynamics including interference, and an irreducible uncertainty in the simultaneous measurement of position and momentum.

Such properties also occur under special circumstances (e.g., low temperatures) in which a particle is trapped in a potential well by a controlling force.

Observing such properties in phenomena governed by the electromagnetic or the weak and strong nuclear forces is common.

But the strength of gravity, many orders of magnitude weaker than the other forces, has not been strong enough to enforce the kind of confinement needed to make quantum reality manifest.

Such an effect has now been seen for the first time: An experiment with ultracold neutrons shows that their vertical motion in Earth's gravitational field comes in discrete sizes.

Physicists at the Institute Laue-Langevin reactor in Grenoble, France, employ a beam of ultracold neutrons. Moving at a pace of 8 m/sec (compared to 300 m/sec for an oxygen molecule at room temperature), the neutrons are sent on a gently parabolic trajectory through a baffle and onto a horizontal plate.

Because the neutrons bounce at such a grazing angle, the plate is essentially a mirror for the neutrons, which are reflected back upwards until gravity saps their ascent; then the neutrons start falling again, eventually to be captured by a detector.

In effect, the neutrons are caught in a vertical potential well: gravity pulls down, while atoms in the surface of the mirror push up.

The researchers report seeing a minimum (quantum) energy of 1.4 picoelectron volts (1.4 x 10^-12 eV), which corresponds to a vertical velocity of 1.7 cm/sec. A comparison of this energy level to the minimum energy for an electron trapped inside a hydrogen atom, -13.6 eV, demonstrates why this kind of detection has not been made before.

The experiment also provides preliminary evidence for higher quantized motion states. In the horizontal direction, there is no confinement and therefore no quantum effect.

(Neutron-interferometry experiments, in which neutron waves are split apart, moved around separate paths and then brought back together to produce an interference pattern, have been influenced by gravity, but these neutron waves were not quantum states owing to the gravitational field. By contrast, the Laue-Langevin experiment is the first to observe quantum states of matter [neutrons] in Earth's gravitational field.)

The next step is to use a more intense beam and an enclosure mirrored on all sides (the energy resolution improves the longer the neutrons spend in the device).

An energy resolution as sharp as 10^-18 eV is expected, which would allow one to test such basic propositions as the equivalence principle, according to which the neutron's gravitational mass (as measured by its free fall in gravity) is the same as its inertial mass (as prescribed by Newton's second law, F=ma, where F is a generic force and a is the acceleration imparted).

(Reference: Nesvizhevsky et al., Nature, 17 Jan 2002.)

(Editor's Note: This article, with some editing, draws upon PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News Number 573, January 16, 2002, by Phillip F. Schewe, Ben Stein, and James Riordon.)